Why is the article lām assimilated in «بِالدِّينِ»?

Al-Māʿūn and Al-Humazah: Applied Assimilation, Concealment, Madd and Hāʾ Rules

• Dāl is a throat letter
• The word has madd yāʾ
• Bāʾ precedes the article
• The following dāl is a sun letter